In order to couple light emitted by a light emitting diode or a semiconductor laser into an optical fiber, such as a single mode fiber, it is well known to use a butt coupling or a lens coupling. The butt coupling is a direct coupling Wherein the fiber is brought close to the light source. The butt coupling provides only about 10% efficiency for a laser, as it makes no attempt to match the mode sizes of the laser and the fiber. For example, the laser mode size may be about 1 μm and the mode size of a single mode fiber may be in the range 6-9 μm. The coupling efficiency may be Improved by tapering the fiber end and forming a lens at the fiber tip.
In a lens coupling design, the coupling efficiency can exceed 70% for a confocal design in which a sphere is used to collimate the laser light and focus it onto the fiber core. The alignment of the fiber core is less critical for the confocal design because the spot size is matched to the fiber's mode size.
These coupling approaches are well suited for excitation of first order modes in optical fibers since the phase of a first order mode does not vary over the fiber cross-section, rather it fits with an electromagnetic field with a symmetric phase and amplitude wave front, such as the electromagnetic field in a light beam emitted by a laser. The phase distribution of a second-order mode, however, is usually symmetric in magnitude but changes sign about a symmetry axis of the fiber, and for higher order modes the phases change sign several times across the cross-section of the fiber. In order to excite higher order modes, the incident electromagnetic beam is typically focused on a part of the cross-section of the fiber, namely an area within which the phase does not change sign. This limits the obtainable coupling efficiency to a value that is roughly equal to the ratio between the illuminated area and the total cross-sectional area of the fiber core. This may be seen by the overlap integral (or inner product) between the two mode plots.
In “Selective launching of higher-order modes into an optical fiber with an optical phase shifter”, by W. Q. Thornburg, B. J. Corrado, and X. D. Zhu, Optics Letters, vol. 19, No. 7, Apr. 1, 1994, a coupling approach is disclosed for exciting a second order mode in a weakly guided, cylindrically symmetric step-index fiber by phase shifting one bisection of the beam so that polarization and phase front of the incident beam matches the desired mode.
As for example disclosed in “Crystal fiber technology”, Jes Broeng, Stig E. Barkou, Anders Bjarklev, Thomas Søndergaard, and Erik Knudsen, DOPS-NYT 2-2000, and “Waveguidance by the photonic band gap effect in optical fibres”, Jes Broeng, Stig E. Barkou, Anders Bjarklev, Thomas Søndergaard, and Pablo M Barbeito, recently, a new approach of making optical fibers has been invented by Professor Philip Russell and his team at the Department of Physics at the University of Bath. In an optical fiber produced according to the new approach, bundles of microscopic dielectric pipes extend along a longitudinal axis of the fiber. Thus, a cross-section of the fiber exhibits holes arranged in an array like atoms in a crystal, hence the name crystal fibers also known as microstructured or holey fibers. The dielectric may be silica, doped silica, polymers, etc.
In index-guided crystal fibers, one or more holes are missing at the center of the array. Without the holes, the glass at the center has a higher density than its surroundings, and light entering the center, i.e. the core, is therefore confined much as it would be in a conventional fiber. The advantage is that the effect is achieved without necessarily having to use two different kinds of glass. An added benefit is that the light can be squeezed into a much narrower core than is the case in conventional fibers, or large mode area single mode fibers can be made. There is a great taylorability of mode size and general mode properties in a photonic crystal fiber (PCF).
In photonic crystal fibers operating by photonic band gap effect (PBG fibers), the holes are arranged in a photonic crystal with band gaps wherein no modes can propagate through tho fiber. By locally breaking the periodicity of the photonic crystal, a spatial region with optical properties different from the surrounding bulk photonic crystal can be created. If such a defect region supports modes with frequencies falling inside the forbidden gap of the surrounding full-periodic microstructure, these modes will be strongly confined to the defect. It is important to note that it is not a requirement that the defect region has a higher index than its surroundings. If the surrounding material exhibits photonic band gap effects, even a low-index defect region is able to confine light and thereby act as a highly unusual waveguide. The defect may be an air filled tube that may provide—in theory—no loss guidance over long distances.
It has been shown that photonic crystal fibers may support single mode operation in a larger wavelength range than conventional fibers. e.g. from UV light to mid-infrared wavelengths, i.e. the entire wavelength range where silica can be used, and that photonic crystal fibers can be designed with a very flat near-zero dispersion over a very broad wavelength range. Further, photonic crystal fibers may be produced with very large positive dispersion for single-mode operation, This may be utilized for dispersion management in fiber systems with negative dispersion or vice versa.
Photonic crystals are structures having a periodic variation in dielectric constant. The dielectric may be silica, doped silica, polymers, etc. By fabricating photonic crystals having specific periodicities, the properties of the photonic band gap can be designed to specific applications. For example, the central wavelength of a photonic band gap is approximately equal to (in order of magnitude) the periodicity of the photonic crystal and the width of the photonic band gap is proportional to the differences in dielectric constant within the photonic crystals, For a general reference, see: J. D. Joannopoulos et al., Photontic Crystals, Princeton University Press, Princeton, 1995. By inclusion of defects with respect to their periodicity in photonic crystals, a localized electromagnetic mode having a frequency within a photonic band gap may be supported. For example, in a three-dimensional photonic crystal formed by dielectric spheres at the sites of a diamond lattice, the absence of a sphere produces a defect. In the immediate vicinity of the absent sphere, the photonic crystal is no longer periodic, and a localized electromagnetic mode having a frequency within the photonic band gap can exist. This defect mode cannot propagate away from the absent void, it is localized in the vicinity of the defect. Thus, the introduction of a defect into the photonic crystal creates a resonant cavity, i.e. a region of the crystal that confines electromagnetic radiation having a specific frequency within the region. A series of defects can be combined to form a waveguide within the photonic crystal. Such waveguides in photonic crystals can include sharp turns, such as 90° bends substantially without loss. For example, U.S. Pat. No. 5,526,449 discloses waveguides based on photonic crystals for incorporation into opto-electronic integrated circuits.
The crystal fibers previously mentioned are examples of two-dimensional photonic crystals with electromagnetic mode supporting defects. A large variety of design options is available to the designer of crystal fibers. By careful selection of preform tube geometry, tube density, tube positions, and utilization of tubes of different types in the same fiber, the designer can provide waveguides with desired characteristics, such as transmission loss, dispersion, non-linearity, mode structure, micro- and macro-bend loss, etc. Examples of various designs are disclosed in WO 99/64903, WO 99/64904, and WO 00/60390.
Examples of one-dimensional photonic crystals are given in U.S. Pat. No. 6,130,780 disclosing an omni-directional reflector with a surface and a refractive index variation along the direction perpendicular to the surface so that a range of frequencies exists defining a photonic band gap for electromagnetic energy incident along the perpendicular direction to the surface The structure further fulfils a criterion by which no propagating states may couple to an incident wave and thus the dielectric structure acts as a perfect reflector in a given frequency range for all incident angles and polarizations.
In WO 00/65386, an all-dielectric coaxial waveguide is disclosed that is designated a coaxial omniguide and that is based on the omni-directional dielectric reflector disclosed in U.S. Pat. No. 6,130,780. The radial confinement of the light in the coaxial omniguide is a consequence of omni-directional reflection and not total internal reflection. This means that the coaxial omniguide can be used to transmit light around much sharper corners than the optical fiber. Also, the radial decay of the electromagnetic field in the coaxial omniguide is much greater than in the case of the optical fiber so that the outer diameter of the coaxial omniguide can be much smaller than the diameter of the cladding layer of the optical fiber without leading to cross-talk.